No one knows exactly how long animals live in a wild state, but records have been kept of animals that have been tamed by men. And scientists are able to come very close to the ages of wild animals by examining the teeth, bones and other parts of the body. Turtles, toads, crocodiles and many reptiles, all cold-blooded animals, have the longest life of land animals, some living up to three hundred years. The elephant is thought to live about one hundred years. Whales live five hundred years, and scientists think some may live to be a thousand years old. Domesticated animals are short lived. A dog is old at twelve, and dies at fifteen. A few have been known to live, but blind and feeble, until twenty. Cats live from thirteen to fifteen years. The wild cats, or lions, live to be forty. Horses and cows are old at twenty, and die before thirty. Of all birds the little singers are the shortest lived, living from three to fifteen years. This may be because of the many accidents that cut their lives short, for pet canaries have lived for twenty-five years. The crow, swan and eagle often live to be a hundred. With a few exceptions, the rule seems to be that the largest animals of all classes, those of which there are fewest in number, live the longest. The same rule seems to apply to plants. There were big trees in California, and whales in the ocean, that were hundreds of years old when Columbus discovered America.